tvix/{ca}store: have some acceptance tests

There's some assumptions on how the different services behave, and we don't really test all of them in unit tests.

We should probably refactor the test suite, to make it easier to extensively test all services. This will get more pressing the more implementations we have

Areas of special interest:

• error handling, ensure that implementations detect invalid data on insert and reject it
• ensure that at least the gRPC implementation detects and rejects invalid data on retrieval (it's a bit hard to feed the others with invalid data to return in first place)

We might want to use something like rstest to define all instances of services to testagainst, and all tests against it (and all variations of it), and then have it emit tests for all possible permutations.